Hydrocarbon treating process having minimum gaseous effluent

ABSTRACT

A process is disclosed for treating hydrocarbon streams for the purpose of removing mercaptans and also converting the remaining mercaptans to disulfides which remain in the hydrocarbon stream being treated. The hydrocarbons rise through a contacting column countercurrently to a descending stream of an aqueous alkaline solution. A limited amount of an oxygen-containing gas is passed into an intermediate point in the column thereby dividing it into an upper sweetening section and a lower extraction section. The flow rate of the oxygen-containing stream is preferably low enough that any gas not consumed in the catalytic oxidation of mercaptans becomes dissolved in the hydrocarbon product stream, and preferably remains dissolved at atmospheric pressure.

FIELD OF THE INVENTION

The invention relates to a process for treating mineral oils such as thetreating processes performed in petroleum refineries to removecontaminants from LPG or naphtha streams. The invention specificallyrelates to the treatment of mercaptan-containing hydrocarbon streams forthe purpose of removing the mercaptans or converting the mercaptans todisulfides. The invention is directly concerned with such treatingprocesses in which an aqueous caustic stream is used to extract themercaptans from the hydrocarbon stream either to remove the mercaptansor as an intermediate step in the oxidation of the mercaptans, therebyforming disulfides which become dissolved in the hydrocarbon phase. Thelatter treating method, which does not reduce the sulfur content of thehydrocarbon stream, is referred to in the petroleum refining arts assweetening.

PRIOR ART

Both the extraction of mercaptans from hydrocarbons through the use ofan alkaline solution and the sweetening of hydrocarbons by the catalyzedoxidation of mercaptans are well known processes. These processes arewidely employed on a large scale in petroleum refineries. In theextraction process, the alkaline solution is regenerated by theoxidation of the dissolved mercaptans to disulfide compounds, which arethen separated from the aqueous solution by decantation. These processesare described in U.S. Pat. Nos. 2,882,224 and 2,921,020. The formerreference is also pertinent for its teaching that the sweeteningoperation may be performed in a countercurrent contacting process.Several forms of this treating process are also shown at page 124 of theApril, 1982 edition of Hydrocarbon Processing.

It is also well known that both extraction and sweetening treating stepsmay be employed in the same process. For instance, the sequentialextraction and sweetening of a sour gasoline is shown at page 224 of theSeptember, 1968 issue of Hydrocarbon Processing. Treating processeswhich employ both extraction and sweetening steps and in which analkaline solution is employed and catalytically regenerated by mercaptanoxidation are shown in U.S. Pat. Nos. 3,409,543 and 3,574,093. Thesepatents are also pertinent for their general teaching as to operatingpractices, process conditions and feedstocks for both sweetening andmercaptan extraction/oxidation operations. It is believed thatheretofore sequential extraction and sweetening steps were performed inseparate zones, and that the same aqueous alkaline solution was nottransferred directly from a sweetening step to an extraction step.

BRIEF SUMMARY OF THE INVENTION

The subject process reduces the capital costs of sweetening andmercaptan extracting of hydrocarbon feed streams. The process alsogreatly reduces or eliminates the discharge of a hydrocarbon-containingvapor stream from a sweetening operation, thereby producing acorresponding reduction in product recovery and pollution controloperating problems of conventional sweetening operations.

A broad embodiment of the invention may be characterized as a processfor treating hydrocarbons which comprises the steps of countercurrentlycontacting a liquid-phase alkaline aqueous stream and a liquid-phasefeed stream comprising mercaptans and hydrocarbons having boiling pointsunder about 650° F. along the height of a unitary vertical contactingzone; and injecting an oxygen-containing stream into an intermediatepoint in the contacting zone, with the added oxygen reacting withmercaptans which still remain in the hydrocarbon-containing stream inthe presence of a mercaptan oxidation catalyst, and thereby effecting asweetening treatment of the feed stream above the point at which theoxygen-containing stream enters the contacting zone and a mercaptanextraction treatment below the point at which the oxygen-containingstream enters the contacting column.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a simplified flow diagram of a preferred embodiment ofthe invention. Numerous pieces of process equipment normally employed insuch a process, including vessel internals, pumps, control systems,etc., have not been shown as they do not directly relate to theinventive concept. This illustration of one embodiment of the drawing isnot intended to preclude from the scope of the subject invention thoseother embodiments set out herein or which result from expected andreasonable modification to those embodiments.

Referring now to the drawing, a feed stream of mercaptan-containingnaphtha from line 1 enters the lower portion of an extraction column orcontactor 2. The naphtha rises upward through the contacting plates ortrays 6 toward the top of the contactor countercurrent to a descendingstream of an aqueous alkaline solution normally referred to as caustic.Approximately half way up the contactor, air is passed into thecontactor through line 4, with the air becoming dissolved in thenaphtha. The naphtha continues upward past the point in the upperportion of the column at which the caustic is added through line 3 andis then removed as a liquid-phase hydrocarbon effluent or product streamthrough line 5. The naphtha has therefore been first treated by theextraction of mercaptans and then further treated by sweetening in whichremaining mercaptans are oxidized to disulfides which remain in thenaphtha.

A resultant mercaptan-rich stream of the aqueous alkaline solution isremoved from the bottom of the contactor through line 7, admixed withair from line 8 and passed into a reactor 10 used as an oxidation zonethrough line 9. The rich alkaline solution is regenerated by theoxidation of mercaptans to disulfides, thereby yielding a mixed-phasereactor effluent carried by line 11 to the phase separator 12. Theremaining nitrogen and any excess oxygen which are not dissolved in theliquids are removed as an off-gas stream discharged through line 13. Thedisulfides are preferably allowed to separate from the nowmercaptan-lean alkaline solution, with the liquid-phase disulfides thenbeing withdrawn through line 14. The regenerated alkaline solution isthen recycled to the contactor through line 3. Alternatively, thedisulfides may be allowed to remain in the regenerated alkalinesolution. In this instance the disulfides also enter the contactor andthen become dissolved in the naphtha of the effluent stream. Thisalternative does not result in a reduction in the sulfur content of thehydrocarbon (naphtha) stream but does produce a sweetened productstream.

DETAILED DESCRIPTION

Treating processes which act upon the mercaptans present in variouspetroleum fractions are employed in virtually every petroleum refinery.Two of the most prevalent types of such treating processes are theextraction of the mercaptans from the hydrocarbon fraction using anaqueous alkaline solution, which is normally referred to simply asextraction, and the catalytic oxidation of the mercaptans to disulfideswhich remain in the hydrocarbon fraction. The latter operation isnormally referred to as sweetening since a successful treating processwill produce a "doctor sweet" product.

In an extraction treating process, the hydrocarbon fraction is broughtinto contact with an aqueous alkaline solution under conditions whichare effective in promoting the transfer of the mercaptans from thehydrocarbon fraction to the alkaline solution. The resultantmercaptan-rich aqueous solution is then separated from the hydrocarbonfraction and regenerated. Extraction therefore decreases the totalsulfur content of the hydrocarbon fraction. Extraction is normally usedto treat the lighter hydrocarbon fractions, such as LPG, which require avery low total sulfur content to meet various product specifications. Asthe average molecular weight of the hydrocarbon fraction increases,there is a proportional increase in the difficulty of removing thedesired amount of mercaptans via extraction with an aqueous alkalinesolution. This is basically due to the fact that the higher molecularweight mercaptans tend to remain in the hydrocarbon phase and do notpreferentially transfer to the aqueous phase to the same extent as lowermolecular weight mercaptans.

The problems of extracting mercaptans from higher molecular weighthydrocarbon fractions are alleviated by the fact that most productspecifications for higher molecular weight petroleum fractions do notpreclude the presence of limited amounts of sulfur. However, sulfur inthe form of mercaptans is normally objectionable even in these heavierhydrocarbons. It has therefore become a customary practice to convertthe mercaptans to disulfides which are allowed to remain in thehydrocarbon stream. Allowing these sulfur-containing compounds to remainin the hydrocarbon fraction also means that the treating process doesnot decrease the quantity of the hydrocarbon fraction. Some hydrocarbonfractions may therefore be sufficiently treated by a simple sweeteningoperation. In other instances, as for example when the hydrocarbonfraction contains a very significant amount of mercaptans, it isnecessary to employ a two-step treating process in which the hydrocarbonfraction is first treated by extraction and is then further treated in asweetening step. The extraction removes the majority of the mercaptansoriginally present in the feed hydrocarbon fraction and the sweeteningstep converts the remaining mercaptans to disulfides.

Although sweetening is widely employed in a highly successful manner,the present higher economic value of hydrocarbons combined with morestringent pollution control regulations has resulted in the occasionaloccurrence of a significant operational problem. More specifically, whenit is desired to sweeten a relatively volatile hydrocarbon fractioncontaining a relatively high amount of mercaptans, the removal orrecovery of the hydrocarbons present in the off-gas of the sweeteningoperation can pose a significant economic burden on an otherwiserelatively inexpensive treating process. More specifically, when it isattempted to sweeten a high mercaptan hydrocarbon such as a naphtha, thequantity of oxygen required for the oxidation of the mercaptans todisulfides exceeds the solubility limits of the oxygen in the hydrogenfraction. Since an excess of oxygen above the stoichiometricallyrequired amount is normally admixed into the hydrocarbon, some of thisoxygen will remain after the sweetening step and is removed as anoff-gas of the sweetening step. As the most economical source of oxygenis air, a much larger quantity of nitrogen than oxygen is charged to thesweetening zone during the sweetening operation. Since the nitrogen isnot consumed in any way during the oxidation of the mercaptans, all thenitrogen present in the air stream except for that which becomesdissolved in the hydrocarbon must also be vented from the sweeteningzone as part of the off-gas stream. This off-gas stream will contain anear equilibrium concentration of the hydrocarbon fraction beingtreated. The recovery of these hydrocarbons from the off-gas streamthrough the use of such means as cryogenic separation or absorptionplaces a heavy economic burden on the treating process. Thesehydrocarbon recovery operations normally require extensive capitalexpenditures and may entail operational systems more complicated thanthe entire treating and alkaline reagent regeneration steps combined. Itis therefore an objective of the subject invention to provide animproved hydrocarbon treating process in which mercaptans are oxidizedto disulfide compounds which remain in the hydrocarbon streams beingtreated. It is a further objective of the subject invention to reduce oreliminated gases discharged from the sweetening zone of a hydrocarbontreating process.

The subject process may be applied to a wide variety of feedhydrocarbons. It may therefore be applied to essentially any hydrocarbonwhich may be treated by sweetening. Treating processes are normallyrestricted to application to those hydrocarbon streams having boilingpoint ranges which fall below 650° F. More preferably the feed stream tothe subject process comprises a mixture of hydrocarbons having boilingpoints below about 430° F., with these boiling point ranges beingdetermined by the appropriate ASTM test method. The feed stream to theprocess may contain low molecular weight hydrocarbons down to andincluding propane and may therefore comprise a mixture of C₃ to C₈hydrocarbons. The preferred feed to the subject process is a naphthastream. Examples of the preferred type of feed hydrocarbon streamtherefore include FCC gasolines, light straight run gasolines and lightcoker naphthas. The subject process is especially suited for treatinghydrocarbons having a relatively high Reid vapor pressure. The feedstream therefore preferably has a Reid vapor pressure above 8 pounds.The feed also preferably has a mercaptan content over 50 ppm and morepreferably over 350 ppm.

In the subject process, the feed stream is charged to the lower portionof a unitary contactor. The feed stream will normally enter thecontactor a short distance above the bottom of the contactor to therebyprovide a settling or separation zone in the bottom of the contactor toallow the separation of entrained hydrocarbon from themercaptan-containing aqueous stream withdrawn at the bottom of thecontactor. The contactor is preferably a single vertical vesselcontaining a sizable number of liquid-liquid contacting trays which maybe of customary design. Such trays are sometimes referred to in the artas jet decks. Although the use of a single vessel contactor is highlypreferred, the use of a contacting zone comprising two or morevertically stacked separate vessels is possible. Another potentialvariation in the structure of the contacting zone or contactor is thesubstitution of a packing material for the preferred liquid-liquidtrays. The contactor or contacting zone is divided into a lowerextraction section and an upper sweetening section at an intermediatelocus at which an oxygen-containing stream is charged to the contactor.It is preferred that both the extraction section and the sweeteningsection contain a sufficient number of liquid-liquid contacting trays orpacking material to provide at least two theoretical extraction units ineach section. More specifically it is preferred that at least fouractual contacting trays are provided above the intermediate point atwhich the oxygen-containing gas stream enters the actuator and at leastfour contacting trays are provided below this intermediate point.

In the subject process, an oxygen-containing stream entering at anintermediate point in the contacting zone supplies the oxygen consumedin the sweetening section of the contacting zone. This oxygen-containingstream could possibly be a liquid phase stream, but it is highlypreferred that a gaseous stream is employed in the process. It is alsohighly preferred that the oxygen-containing stream is a stream of air,although oxygen-enriched air or pure oxygen could be employed if sodesired. It is also highly preferred that the total amount of gaspresent in the oxygen-containing stream becomes dissolved in the totalliquids present in the contacting zone. Specifically it is preferredthat the rate of addition of all the gaseous compounds present in theoxygen-containing stream is limited to a quantity which is below theremaining gas solubility capacity of the feed hydrocarbon stream. Thissolubility limit will vary depending on such factors as the compositionof the feed hydrocarbon, the temperature of the feed hydrocarbon as itpasses through the sweetening section of the contacting zone, thepressure at which the process is being operated, etc. It is very highlypreferred that the rate of gas addition is low enough that nosignificant amount of the remaining added gas(es) will be released whenthe product hydrocarbon is stored at atmospheric pressure. Therefore inthe preferred embodiments of the process, the hydrocarbons rising abovethe sweetening section of the contacting zone enter a liquid-liquidphase separation zone located in the upper part of the contacting zoneand are then removed as a totally liquid phase stream from the top ofthe contacting zone. Ideally, no vaporous material will accumulate inthe upper portion of the contacting zone or be removed in admixture withthe treated product stream. However as a safety precaution and to allowfor temporary misoperation or process upsets, the hydrocarbon effluentstream could be routed through a vapor-liquid separation zone designedto trap any vaporous material emerging with the hydrocarbon effluentstream. When such a separation zone would be employed, there wouldnormally be no flow of gaseous material from the separator. The treatedhydrocarbon effluent stream may be passed through the customaryfinishing steps such as sand filters, etc.

One embodiment of the subject process may be broadly characterized as aprocess for treating hydrocarbons which comprises the steps of passing aliquid feed stream comprising hydrocarbons having boiling points belowabout 600° F. and mercaptans into a lower portion of a unitarycontacting column, with the feed stream rising upward through thecolumn; passing a stream of an aqueous alkaline solution into an upperportion of the column, with the aqueous alkaline solution passingdownward through the column countercurrent to rising hydrocarbons;passing a first oxygen-containing gas stream into an intermediate pointof the column, with oxygen from the gas stream reacting with mercaptansin the presence of a mercaptan oxidation catalyst; removing ahydrocarbon effluent stream comprising disulfide compounds from an upperpoint in the column above the level at which the stream of aqueousalkaline solution is passed into the column; and removing a stream of anaqueous alkaline solution comprising extracted mercaptans from a lowerpoint in the column below the level at which the gas stream is passedinto the column.

Since it is preferred to avoid having vapor present in the hydrocarboneffluent stream, the amount of sweetening which may be performed in theupper or sweetening section of the contactor is limited by thesolubility of the residual gases in the hydrocarbon stream. Therefore,unless pure oxygen is employed and totally reacted within the sweeteningzone, a condition which is not achieved in commercial operation, only alimited mercaptan concentration may be converted to disulfides in thesweetening zone. The remaining portion of the mercaptans present in thefeed stream must be removed through the extraction treating stepperformed below the sweetening zone. The flow rate of the alkalinesolution must therefore be sufficient to remove that quantity of theentering mercaptans which cannot be treated in the sweetening zone. Theamount of alkaline solution circulated through the extraction sectionmay exceed that of the sweetening section. For instance, a portion ofthe alkaline solution withdrawn from the bottom of the contactor (vialine 7) may be returned at a point below the entrance of the air stream.

The extracted mercaptans enter the aqueous alkaline solution and arethen subsequently converted to disulfides in a manner similar to theknown regeneration techniques commercially employed for this purpose. Aprocess flow similar to that illustrated in the drawing is preferablyemployed for this purpose. In this regeneration procedure, themercaptan-containing aqueous alkaline solution is admixed with air andpassed through a reactor or oxidizer which may contain a fixed bed ofmercaptan oxidation catalyst. Alternatively, the mercaptan oxidationcatalyst which is dissolved in the aqueous alkaline solution for thepurpose of promoting the mercaptan oxidation which occurs in thesweetening section may be the sole means of oxidation catalysis employedin the reactor. When correctly performed, this oxidative regenerationresults in the production of a mixed phase effluent which is passed intoa separator. The residual nitrogen which remains from the air streamused to supply oxygen along with residual oxygen is removed as a gasstream from the separator. Since the feed hydrocarbons are not presentin this separator, this gas stream will not contain the feedhydrocarbons and will contain only a very limited amount of disulfides.The disulfides have a limited solubility in the aqueous alkalinesolution normally employed in the process and may therefore be separatedby decantation as a less dense "hydrocarbon phase" which is commonlyreferred to as a disulfide oil. In an alternative embodiment of thesubject process, the disulfides are not separated from the aqueousalkaline solution but are returned to the top of the contactor as partof the alkaline solution. The disulfides are normally soluble in thefeed hydrocarbons and will therefore be extracted from the alkalinesolution by the hydrocarbon stream being treated. This will transfer thedisulfides to the hydrocarbon stream and they are then removed as acomponent of the hydrocarbon effluent stream of the contactor. Thisalternative embodiment results in the hydrocarbon effluent stream havinga total sulfur content close to that of the feed stream. However, theproduct stream is "sweet" and will meet product specifications callingfor a sweet product.

The subject extraction process may utilize in the alkaline solution anyalkaline reagent which is capable of extracting mercaptans from the feedstream at practical operating conditions and which may be regenerated inthe manner described. A preferred alkaline reagent comprises an aqueoussolution of an alkaline metal hydroxide, such as sodium hydroxide orpotassium hydroxide. Sodium hydroxide, commonly referred to as caustic,may be used in concentrations of from 1 to 50 wt.%, with a preferredconcentration range being from about 5 to about 25 wt.%. Optionally,there may be added an agent to increase the solubility of the mercaptansin the alkaline solution.

The conditions employed in the contacting zone may vary greatlydepending on such factors as the nature of the hydrocarbon stream beingtreated and its mercaptan content, etc. In general, both extraction andsweetening may be performed at an ambient temperature above about 60° F.and at a pressure sufficient to ensure liquid state operation. Theoperating pressure may range from atmospheric up to 1000 psig or more,but a pressure in the range of from about 60 to about 350 psig ispreferred. The temperature in the contacting zone is normally confinedwithin the range of 50° to about 250° F., preferably from 80° to 120° F.The ratio of the volume of the alkaline solution required in theextraction section per volume of the feed stream will vary depending onthe mercaptan content of the feed stream. Normally this ratio will bebetween 0.01:1 and 1:1, although other ratios may be desirable. The rateof flow of the alkaline solution will typically be about 1 to 10% of therate of flow of an LPG stream and may be up to about 20% of a lightstraight run naphtha stream. These rates may be obtained in various waysas set out herein. The extraction section of the contactor preferablycontains trays having a large number of circular perforations. Optimumextraction in this liquid system is obtained with a velocity through theperforations of from about 5 to about 10 feet per second. As previouslymentioned, packing and other types of extraction equipment could beemployed if desired. Preferably at least one-half of the extractablemercaptans should be transferred to the alkaline solution from the feedstream within the extraction section of the contacting zone.

Proper operation of the extraction section results in the formation of amercaptan-containing alkaline stream which is also referred to as a richalkaline stream or rich caustic stream. This stream is removed from thecontacting zone and then mixed with an air stream supplied at a ratewhich supplies at least the stoichiometric amount of oxygen necessary tooxidize the mercaptans in the alkaline stream. The air or otheroxidizing agent is well admixed with the liquid alkaline stream and themixed-phase admixture is then passed into the oxidation zone. As alreadypointed out, the oxidation of the mercaptans is promoted through thepresence of a catalytically effective amount of an oxidation catalystcapable of functioning at the conditions found in the reactor oroxidizing zone. Several suitable materials are known in the art.Preferred as a catalyst is a metal phthalocyanine such as cobaltphthalocyanine or vanadium phthalocyanine, etc. Higher catalyticactivity may be obtained through the use of a polar derivative of themetal phthalocyanine, especially the monosulfo, disulfo, trisulfo andtetrasulfo derivatives.

The preferred mercaptan oxidation catalysts may be utilized in a formwhich is soluble or suspended in the alkaline solution or it may beplaced on a solid carrier material. If the catalyst is present in thesolution, it is preferably cobalt or vanadium phthalocyanine disulfonateat a concentration of from about 5 to 1000 wt. ppm. If the catalyst ispresent in the alkaline solution, then the same catalyst is employed inboth the sweetening section of the contacting zone and in theregeneration of the rich alkaline solution. If supported catalyst isemployed, then the same or different catalysts may be used in these twolocations. Carrier materials should be highly absorptive and capable ofwithstanding the alkaline environment. Activated charcoals have beenfound very suitable for this purpose, and either animal or vegetablecharcoals may be used. The carrier material is to be suspended in afixed bed which provides efficient circulation of the alkaline solution.Preferably the metal phthalocyanine compound comprises about 0.1 to 2.0wt.% of the final composite. More detailed information on liquid-phasecatalysts and their usage may be obtained from U.S. Pat. Nos. 2,853,432and 2,882,224. Likewise, further information on fixed bed operations iscontained in U.S. Pat. Nos. 2,988,500, 3,108,081 and 3,148,156.

The oxidation conditions utilized for regeneration of the rich alkalinesolution include a pressure of from atmospheric to about 1000 psig, andpreferably are substantially the same as used in the downstream phaseseparation zone. This pressure is normally less than 75 psig. Thetemperature may range from ambient to about 200° F. when operating nearatmospheric pressure and to about 400° F. when operating atsuperatmospheric pressures. In general, it is preferred that atemperature within the range of about 100° to about 175° F. is utilized.The reactor or oxidation preferably contains a packed bed to ensureintimate mixing. This is done in all cases, including when the catalystis circulated within the alkaline solution.

The phase separation zone which receives the regenerated alkalinesolution may be of any suitable configuration, with a settler such asrepresented in the drawing being preferred. A simple gas separationvessel may be employed if all of the liquid material is to be passedinto the contacting zone. There is formed in this zone a first liquidphase containing the aqueous alkaline solution and a second liquid phasecontaining the disulfide compounds. The phase separation zone is sizedto allow the denser alkaline solution to separate by gravity from thedisulfide compounds. This may be aided by a coalescing means located inthe zone. Normally, a residence time in excess of 90 minutes isprovided. A stream of a suitable hydrocarbon, such as a naphtha, is insome instances admixed with the material entering the zone to aid in theseparation of the two liquid materials. The disulfide compounds and anyadded hydrocarbons are removed from the process as a by-product stream,and the aqueous alkaline solution is withdrawn for passage into thecontacting zone.

It is desirable to run the phase separation zone at the minimum pressurewhich other design considerations will allow. This is to promote thetransfer of the excess oxygen, nitrogen and water into the vapor phase.The pressure in the phase separation zone may range from atmospheric toabout 300 psig or more, but a pressure in the range of from about 10 to50 psig is preferred. The temperature in this zone is confined withinthe range of from about 50° to about 250° F., and preferably from about80° to 130° F.

The excess oxygen admixed with the alkaline solution during regenerationresults in the presence of unused gaseous oxygen in the phase separationzone. This, along with the nitrogen from the air and some water vapor,is removed as a relatively small vapor stream. The presence of oxygenvapor in any refinery process stream calls for the utmost care inpreventing the accidental formation of explosive mixtures by theoxygen-containing stream becoming admixed with hydrocarbons or othercombustibles. It is therefore the standard practice to purposely admixthis stream with a stream of volatile hydrocarbons having a sufficientflow rate to establish a hydrocarbon concentration above the explosivelimit in the resulting mixed gas stream. In this way, any accidentaladmixture of the separator off-gas stream with hydrocarbons only resultsin a further enrichment of the stream in hydrocarbons and cannot lead toan explosive mixture. The vapor stream used for this purpose ispreferably a fuel gas stream, that is, one which is scheduled forcombustion, and the resulting admixture is used as fuel.

Excess water produced in the process may be removed from the alkalinesolution by contacting a relatively small portion of the regeneratedsolution with a vapor stream under conditions which promote the transferof water into the vapor stream from the alkaline solution. Althoughother gas streams could be used, it is greatly preferred that the vaporstream used for removing water from the alkaline solution is the samevapor stream which is subsequently admixed with the phase separationzone off-gas stream to increase the hydrocarbon content of that stream.The vapor stream used in the contacting step preferably is rich involatile hydrocarbons, that is, hydrocarbons having fewer than sixcarbon atoms per molecule. The relatively small alkaline solution streamand the vapor stream are brought together in a contacting zone which isalso referred to as a water balance column. Details on the operation ofa water balance column are available in the patent literature such asU.S. Pat. Nos. 4,104,155 and 4,362,614.

Although it is preferred that the mercaptan oxidation catalyst employedin the sweetening section is contained in the aqueous stream, a solidoxidation catalyst can be present in the sweetening section. This isespecially true when a packed bed sweetening section is utilized, sincethe catalyst may form some or all of the packing material. Anothervariation in the subject process comprises splitting the flow of theaqueous alkaline solution into two portions, with the first portionentering the top of the sweetening section in the manner previouslydescribed and with a second portion entering the contacting column atsome point within or just above the extraction section. This mode ofoperation can provide high rates of extraction in the extraction sectionwithout requiring high flow rates of the aqueous stream through thesweetening section. Therefore from about 20 to about 80 volume percentof the total amount of the aqueous alkaline solution which is passedinto the contacting column may enter the column at an intermediate pointjust above the extraction section and below the sweetening section.

I claim as my invention:
 1. A process for treating hydrocarbons whichcomprises the steps of:(a) countercurrently contacting a liquid-phasealkaline aqueous stream and a liquid-phase feed stream comprisingmercaptans and hydrocarbons having boiling points under about 650° F.along the height of a vertical contacting zone; and, (b) injecting anoxygen-containing stream into an intermediate point in the contactingzone, with the oxygen reacting with mercaptans in the presence of amercaptan oxidation catalyst, and thereby effecting a sweeteningtreatment of the feed stream above the point at which theoxygen-containing stream enters the contacting zone and a mercaptanextraction treatment of the feed stream below the point at which theoxygen-containing stream enters the contacting zone.
 2. The process ofclaim 1 further characterized in that the mercaptan oxidation catalystcomprises a metal phthalocyanine.
 3. The process of claim 2 furthercharacterized in that all gas present in the oxygen-containing streambecomes dissolved in the liquid present in the contacting zone.
 4. Theprocess of claim 3 further characterized in that the oxygen-containingstream is a gas stream comprising air.
 5. The process of claim 2 furthercharacterized in that the mercaptan oxidation catalyst is dissolved inthe alkaline aqueous stream.
 6. The process of claim 5 furthercharacterized in that the contacting zone is a single column whichcontains at least four contacting trays above and at least fourcontacting trays below the intermediate point at which theoxygen-containing stream is passed into the contacting column.
 7. Theprocess of claim 4 further characterized in that the feed stream has aReid vapor pressure above 8 lbs.
 8. A process for treating hydrocarbonswhich comprises the steps of:(a) passing a liquid feed stream comprisinghydrocarbons having boiling points below about 600° F. and mercaptansinto a lower portion of a unitary contacting column, with the feedstream rising upward through the column; (b) passing a stream of anaqueous alkaline solution into an upper portion of the column, with theaqueous alkaline solution passing downward through the columncountercurrent to rising hydrocarbons; (c) passing a firstoxygen-containing gas stream into an intermediate point of the column,with oxygen from the gas stream reacting with mercaptans in the presenceof a mercaptan oxidation catalyst; (d) removing hydrocarbon effluentstream comprising disulfide compounds from an upper point in the column;and, (e) removing a stream of an aqueous alkaline solution comprisingextracted mercaptans from a lower point in the column below the level atwhich the gas stream is passed into the column.
 9. The process of claim8 further characterized in that the flow rate of the firstoxygen-containing gas stream is such that all of the added gas becomesdissolved in liquids present in the column.
 10. The process of claim 9further characterized in that the oxidation catalyst is dissolved in theaqueous alkaline solution.
 11. The process of claim 10 furthercharacterized in that the hydrocarbons of the feed stream have boilingpoints below about 430° F.
 12. The process of claim 11 furthercharacterized in that the oxidation catalyst comprises a metalphthalocyanine.
 13. The process of claim 12 further characterized inthat the extraction column contains liquid-liquid extraction trays. 14.The process of claim 12 further characterized in that the extractioncolumn contains a solid high surface area packing material.
 15. Theprocess of claim 14 further characterized in that the packing materialis preferentially wetted by the aqueous alkaline solution.
 16. Theprocess of claim 10 further characterized in that the stream of aqueousalkaline solution removed from the column is admixed with a secondoxygen-containing gas stream under conditions effective to promote theoxidation of mercaptans in the aqueous alkaline solution to disulfides,and any undissolved gas is then separated from the resultant admixtureof disulfides and aqueous alkaline solution.
 17. The process of claim 16further characterized in that the resultant admixture of disulfides andaqueous alkaline solution is passed into the extraction column as saidstream of aqueous alkaline solution, with disulfides present in theresultant admixture thereby becoming dissolved in the hydrocarboneffluent stream.
 18. The process of claim 9 further characterized inthat the feed stream has a mercaptan concentration over 50 ppm.